US 7411713 B2
A micromirror array 110 fabricated on a semiconductor substrate 11. The array 110 is comprised of four operating layers 12, 13, 14, 15. An addressing layer 12 is fabricated on the substrate. A raised electrode layer 13 is spaced above the addressing layer by an air gap. A hinge layer 14 is spaced above the raised electrode layer 13 by another air gap. A mirror layer 15 is spaced over the hinge layer 14 by a third air gap.
1. A method of forming an array of micromirror elements, comprising the steps of:
depositing a first spacer layer on a substrate;
patterning the first spacer layer to define lower hinge support vias and raised electrode vias;
depositing a raised electrode layer over the first spacer layer;
patterning the raised electrode layer to form at least two raised electrodes associated with each micromirror elements;
depositing a second spacer layer over the patterned raised electrodes;
patterning the second spacer layer to form hinge support vias over the lower hinge support vias;
depositing a hinge layer over the second spacer layer;
patterning the hinge layer to form at least one hinge associated with each micromirror element;
depositing a third spacer layer over the hinge layer;
patterning the third spacer layer to define mirror support vias;
depositing a mirror layer over the third spacer layer;
patterning the mirror layer to form an array of micromirrors; and
removing the first, second, and third spacer layers.
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This application is a divisional of application Ser. No. 10/345,586, filed Jan. 16, 2003 now U.S. Pat. No. 7,088,486 which claims the benefit of provisional Application No. 60/353,979, filed Jan. 31, 2002.
This application is related to co-pending application Ser. No. 10/298,423 filed Nov. 21, 2001 entitled “Yokeless Hidden Hinge Digital Micromirror Device” now U.S. Pat. No. 7,011,415.
This invention relates to micro-electromechanical devices and their fabrication, and more particularly to a digital micromirror device having an improved design.
A Digital Micromirror Device™ (DMD™) is a type of microelectromechanical systems (MEMS) device. Invented in 1987 at Texas Instruments Incorporated, the DMD is a fast, reflective digital light switch. It can be combined with image processing, memory, a light source, and optics to form a digital light processing system capable of projecting large, bright, high-contrast color images.
The DMD is fabricated using CMOS-like processes over a CMOS memory. It has an array of individually addressable mirror elements, each having an aluminum mirror that can reflect light in one of two directions depending on the state of an underlying memory cell. With the memory cell in a first state, the mirror rotates to +10 degrees. With the memory cell in a second state, the mirror rotates to −10 degrees. By combining the DMD with a suitable light source and projection optics, the mirror reflects incident light either into or out of the pupil of the projection lens. Thus, the first state of the mirror appears bright and the second state of the mirror appears dark. Gray scale is achieved by binary pulsewidth modulation of the incident light. Color is achieved by using color filters, either stationary or rotating, in combination with one, two, or three DMD chips.
DMD's may have a variety of designs, and the most popular design in current use is a structure consisting of a mirror that is rigidly connected to an underlying yoke. The yoke in turn is connected by two thin, mechanically compliant torsion hinges to support posts that are attached to the underlying substrate. Electrostatic fields developed between the underlying memory cell and the yoke and mirror cause rotation in the positive or negative rotation direction.
The fabrication of the above-described DMD superstructure begins with a completed CMOS memory circuit. Through the use of six photomask layers, the superstructure is formed with alternating layers of aluminum for the address electrode, hinge, yoke, and mirror layers and hardened photoresist for sacrificial layers that form air gaps.
One aspect of the invention is an array of digital micromirror pixel elements. The array has a structure defined by three layers spaced from each other with an air gap between each layer. A mirror layer has a reflective mirror associated with each pixel element. A hinge layer is spaced under the mirror layer, and has a torsion hinge under each mirror and attached to the mirror such that the mirror may tilt above the hinge layer. The hinge layer further has spring tips under each mirror and mechanically connected to the address layer. An outboard electrode layer has raised outboard electrodes for providing electrostatic attraction to the underside of each mirror. An address layer is spaced under all these layers, and has circuitry for controlling operation of the pixel elements.
As stated in the Background, conventional DMD designs have a yoke under the mirror, at the hinge level. During operation, the yoke rather than the mirror lands on an underlying landing surface. The yoke has spring tips, used for landing. In the present invention, there is no yoke and the mirror lands on spring tips that are stationary under the mirror at the hinge level. At the same time, the outboard electrode layer provides a sufficiently large air gap under the mirror so as to permit proper operation of the DMD.
U.S. Pat. Ser. No. 10/298423, referenced above, describes a yokeless hidden hinge DMD design, having three layers. The middle layer comprises a hinge, spring tips, and raised electrodes. This layer is sometimes referred to as a “binge” layer because it replaces the yoke and beam layers of previous designs. The present invention is directed to a four layer, “double binge”, design, in which a new layer, lower than the hinge and spring tips, has an additional raised “outboard” electrode.
DMD pixel element 10 is a monolithically integrated MEMS superstructure cell fabricated over a CMOS SRAM cell 11. Three sacrificial layers (see
The uppermost metal layer 15 has a reflective mirror 15 a. The air gap under the mirror 15 a frees the mirror 15 a to rotate about a compliant torsion hinge 14 a, which is part of the second metal layer 14. A third metal layer 13 comprises two raised outboard electrodes 13 a. A fourth metal (M3) layer 12 has address electrodes 12 a for the mirror 15 a, the address electrodes 12 a being connected to SRAM cell 11. The M3 layer 12 further has a bias bus 12 b, which interconnects the mirrors 15 a of all pixels to a bond pad at the chip perimeter. An off-chip driver supplies the waveforms necessary for proper digital operation.
The DMD mirrors 15 a are each 16 um square and made of aluminum for maximum reflectivity. They are arrayed on 17 um centers to form a matrix having a high fill factor (˜90%). The high fill factor produces high efficiency for light use at the pixel level and a seamless (pixelation-free) projected image. The placement of hinge layer 14 under the mirrors 15 a permits a close spacing of the mirrors 15 to each other. Because of the underlying placement of the hinges, an array of pixel elements 10 is referred to as a “hidden hinge” type DMD architecture.
In operation, electrostatic fields are developed between the mirror 15 a and its address electrodes, which include the electrodes 12 a on layer 12 as well as the raised electrodes 13 a on layer 13. As explained below and as illustrated in
A feature of the invention is the increase in the distance between hinge layer 14 and the address layer 12, which is provided by the addition of layer 13. This distance enhances proper operation of hinge 14 a. It also permits the spacing of the distance between the underside of mirror 15 a and the raised electrodes 13 a to be optimized. Specifically, layer 13 permits the spacing of these various elements to be such that electrostatic torque is optimized but without air damping that would be caused by too small a gap under hinge 14 a. It also results in quick settling times for mirrors 15 a upon touchdown.
As shown in
Through the use of photomasking techniques, the M3 layer 12 is formed above the CMOS 11. This M3 layer 12 is formed with aluminum for address and bus circuitry. The aluminum is sputter-deposited and plasma-etched using plasma-deposited SiO2 as the etch mask. M3 layer 12 may be etched in a pattern used for DMD structures previously described in U.S. Pat. No. 6,028,690, entitled “Reduced Micromirror Gaps for Improved Contrast Ratio, and in U.S. Pat. No. 5,583,688, entitled “Multi-level Digital Micromirror Device”, both assigned to Texas Instruments Incorporated. These patents are incorporated by reference herein.
A spacer layer 21, identified as S1, is then deposited over the M3 layer 11. Spacer layer 21 may be formed from hardened photoresist. Later in the packaging flow, this spacer layer 21 is plasma-ashed to form an air gap, but leaving the support posts shown in
In operation, the raised electrodes 13 a are in electrical communication with the address electrodes 12 a on layer 12, such as by means of vias. However, in other embodiments, layer 13 may be patterned such that raised electrodes on layer 13 carry voltages from different sources, such as bias or reset voltages from bias/reset bus 12 b.
The packaging flow begins with the wafers partially sawed along the chip scribe lines to a depth that will allow the chips to be easily broken apart later. Before separating the chips from one another, each chip is tested for full electrical and optical functionality by a high-speed automated wafer tester. The chips are then separated from the wafer, and proceed to a plasma etcher that is used to selectively strip the organic sacrificial layers, S1 and S2, from under the mirror layer 15 and hinge layer 14. The chips are then plasma-cleaned, relubricated, and hermetically sealed in a package.
Although the present invention has been described in detail, it should be understood that various changes, substitutions, and alterations can be made hereto without departing from the spirit and scope of the invention as defined by the appended claims.